The present invention relates generally to web control systems and, more particularly, to a dual mode control system for controlling the lateral position of a moving web.
The control of moving webs of material is important to numerous production processes. For example, in the packaging industry, material which may ultimately be formed into container cartons and the like is initially operated on in web form. Typical production operations in the packaging industry may include the printing of a repeating series of graphics on a moving web of material; the lamination of one moving web of material to another moving web of material; the creasing or cutting of predetermined portions of a moving web of material to form fold lines, etc.; and the final cutting of a moving web of material to form carton blanks or the like. In such operations, it is necessary to accurately control the lateral position of the moving webs to ensure that the printing, lamination, cutting, etc. occur at the correct position on the webs. During such production operations, web speeds may be on the order of 1000 feet per minute. At such speeds lateral misalignments for even a few minutes may result in significant and costly amounts of scrap. Thus, there is a need in such high speed operations to provide a web lateral guidance system which is capable of reacting relatively quickly to alignment errors to prevent scrap. Physical edge sensing control systems are often used in such high speed production. A commercially available edge sensing control system capable of rapid response to lateral alignment errors is manufactured by the Fife Corporation of P.0. Box 26508, Oklahoma City, Okla. 73126, and sold commercially under the name of Auto Center Control System Model PE1-11 and PE1-11L. However, a problem that exists with such rapid response edge control systems is that such systems are not suited to making extremely precise adjustments. Another precision related problem, which occurs when such edge control systems are used to control a moving web having graphics printed thereon, is caused by the fact that the relative lateral position of the graphics on a web may vary slightly from roll to roll of web material and may even vary slightly within a single roll. Thus, in situations where web lateral position is to be determined based on the position of web graphics rather than the actual physical position of the web edge, it is often preferable to use a sensor which directly senses the graphics position. An optical sensor such as Model 2201 Line Guiding System which is commercially available from the Fife Corporation of P.0. Box 26508, Oklahoma City, Okla. 73126, may be used for sensing web graphics. Such a sensor generally has a very narrow scan path on the order of 0.05 inches which is typically focused on a longitudinally extending black guide line associated with the graphics printed on a web. Such a control system is capable of providing extremely accurate control of the web so long as the associated black line falls within its scan path. However, when a gross error is encountered in the web alignment, such as may be caused by a splice in the web, such a system becomes "lost." Loss of the guide line from the scan path of such a system generally causes the production operation to terminate and requires that an operator manually realign the web with the optical scanner. A need exists for a lateral control system which is capable of precise control under normal conditions and which is capable of rapidly reestablishing control when gross errors occur.